[clang] [llvm] [Clang][AST][NFCI] Make Decl reference attributes directly (PR #219138)

[Richard Smith via llvm-commits]

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@@ -355,6 +351,13 @@ class alignas(8) Decl {
   LLVM_PREFERRED_TYPE(Linkage)
   mutable unsigned CacheValidAndLinkage : 3;
 
+  /// The attributes attached to this declaration, or null if it has none.
+  ///
+  /// This pointer is the sole record of whether the declaration has
+  /// attributes, so it must be cleared whenever the vector becomes empty.
+  /// Owned by the ASTContext that allocated it.
+  AttrVec *Attrs = nullptr;
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zygoloid wrote:

As a cheap experiment, can you try putting back a padding bit where the old `hasAttr` bit was and rerunning a comparison against the baseline? (So we're measuring map versus storage-in-Decl, and not treating the Decl storage as free.) That should give us a quick idea of what proportion of Decls actually end up with a non-null attribute pointer, without "cheating" by reusing the padding bits in Decl that are only there by accident.

https://github.com/llvm/llvm-project/pull/219138